If the emotional memory of a traumatic car accident or the thrill of first love are remembered with a special resonance, it is because they engage different brain structures than do normal memories, say Duke researchers reporting in a June issue of the journal Neuron.
Their new study provides clear evidence from humans that the brain's emotional center, called the amygdala, interacts with memory-related brain regions during the formation of emotional memories, perhaps to give such memories their indelible emotional resonance.
The study by Florin Dolcos, Kevin LaBar, and Roberto Cabeza was reported in June in the journal Neuron. Dolcos is a research associate in the Brain Imaging and Analysis Center; LaBar and Cabeza are assistant and associate professors of psychological and brain sciences, respectively. They are also on the faculty of the Center for Cognitive Neurosciences. Their research was supported by the National Institutes of Health.
According to Dolcos, the researchers were seeking evidence for the "modulation hypothesis"--evidence that the brain's emotional region modulates activity in the memory regions to form an emotional memory. "This idea was supported by animal research, but the evidence from neurologically intact humans was scarce and indirect. So, our goal was to find the right method that would allow us to demonstrate that this phenomenon happens in humans, too."
The researchers exposed volunteer subjects to a slideshow of both positive and negative images. The negative images were a grisly display of aggressive acts and injured people; the positive pictures presented the viewer with scenes of romance or sporting triumph. Neutral pictures were also part of the slideshow: a building, a person shopping in a mall.
Throughout the slideshow, participants' brain activity was monitored using functional magnetic resonance imaging that measured blood flow to different regions of the brain. Following the show, the researchers tested participants' memory of the images they viewed.
As expected, says Dolcos, "we found evidence that the interaction between the emotional and memory regions occurred more systematically and consistently during the formation of emotional memories than during the formation of neutral memories. And we found that the subjects showing greater successful encoding activity in the emotional region also had greater activity in the memory regions."
According to Cabeza, "We also found indications that some regions within the medial temporal lobe may actually be more specialized for encoding neutral information. We don't know exactly what the processes involved are, or why these regions are engaged. But we speculate that the regions that were more activated for emotional stimuli are involved in semantic processing of the meaning of the images, whereas those that are more activated by neutral stimuli reflect perceptual processing."
The findings not only establish the functional link between the emotional and memory areas, says Cabeza, but also hint at differences within the memory areas that should be the subject of further studies. As part of their research, the authors are now exploring the role of these brain regions during the retrieval of well-consolidated emotional memories.
Cabeza says that better delineation of the role of the amygdala in emotional memory could aid understanding of post-traumatic stress disorder--especially such phenomena as flashbacks of traumatic memories.